Various electronic circuits require large resistance values. For example, many low-dropout regulator (LDO) circuits include an internal reference filter with a large continuous-time time constant to reduce the l/f noise of an associated internal reference voltage source. These large continuous-time time constants can require very large resistance values (e.g., 500 MΩ to 5 GΩ, etc.).
FIG. 1 illustrates generally an existing resistor-capacitor (RC) circuit 100 including a filter resistor (RFILT) composed of first, second, third, and fourth resistors 111, 112, 113, 114, each having a unit value (RUNIT), and a filter capacitor (CFILT) 120. However, if the filter resistor is a standard high-ohmic poly resistor, it would require a large amount of area on silicon. Accordingly, there is a need to provide large resistance values using a smaller area than traditional resistors.
FIG. 2 illustrates generally an example active resistor solution 500, including first and second PMOS transistors 150, 160, a bias current 155, a capacitor 165, an input (IN), and an output (OUT). The first and second PMOS transistors 150, 160 are configured as a PMOS current mirror with the bias current 155. The gate of the first PMOS transistor 150 is coupled to the drain, such that the first PMOS transistor 150 is connected as a diode. The drop from the source of the first PMOS transistor 150 to the gate of the first PMOS transistor 150 provides a constant gate-to-source voltage (VGS) for the second PMOS transistor 160 to enable the second PMOS transistor 160 to provide a near constant resistance between the input and the output across a wide range of positive input values. However, the resistance of the active resistor solution 500 is entirely dependent on the voltage drop across the first PMOS transistor 150 and the electrical characteristics of the second PMOS transistor 160 at that gate-to-source voltage (VGS). Accordingly, it can be difficult to easily provide a specific or controllable resistance value.